The present invention relates generally to a method for the hydrogenation or liquefaction of coal, and more particularly to a method of continuously converting coal particulates to liquid hydrocarbon products.
In continuous direct coal liquefaction processes, pulverized coal is fed as a slurry including a portion of the coal liquefaction product. The coal slurry is usually pressurized by a pump, passed through a preheater and then into a reactor where the coal is converted to liquid and gaseous products under the influence of elevated temperature and hydrogen pressure. The maximum solids content of the feed slurry in such previous processes is limited to slightly more than 40 weight percent since with greater solids content the slurry becomes so viscous as to be unpumpable. The use of such relatively large concentrations of solvent with the feed coal is disadvantageous in that it is wasteful of energy since the solvent must be heated to reaction temperature along with the coal and is also wasteful of the interior volume of process equipment in that only about 50 percent or less by volume of the feed slurry consists of coal. Thus, it is desirable to have an alternative method to the conventional coal/solvent slurry for feeding coal to a liquefaction process.
It is also known within the art that an efficient method for introducing catalysts into a coal liquefaction system is through impregnation from solution. The catalysts are often salts of transition metals such as iron, molybdenum, nickel or tin. If water-soluble salts are used, the catalysts may be impregnated from an aqueous solution. Compared to other methods of catalyst addition, impregnated catalysts are efficacious at small concentrations, which is thought to be due to the highly dispersed nature of the catalysts within the coal particles and the proximity of catalysts to reactive sites in the coal.
The direct liquefaction of coal in the presence of large amounts of water is also known in the art. Batchwise reactions have been carried out using ratios of coal-to-water that could be obtained from a pumpable slurry of coal and water. A major disadvantage to carrying out liquefaction in the presence of large quantities of water is the contribution of the vapor pressure of water to the total system pressure. Typically such liquefaction is carries out at a total pressure in the vicinity of 4,000 psig or higher which is not economically attractive.
In U.S. Pat. No. 4,735,706 to the inventor, Ruether, coal-oil agglomerates are prepared in an aqueous slurry and the resulting mixture pumped to reaction pressure where excess water and unagglomerated solids are removed. Sufficient catalyst, previously dissolved in the aqueous solution, remains deposited on the agglomerates entering the liquefaction reactor. Although this process is effective and useful it has the disadvantage of requiring dewatering and drying at elevated pressures which can substantially increase process equipment cost. This prior patent is incorporated by reference herein for describing the background of the present invention, the preparation of coal-oil agglomerates and for other purposes as will be seen below.